The present invention relates generally to digital modulators, and more specifically to a base-station digital modulator for mobile communications system in which the transmission power of the base station is varied to compensate for variations in distance to mobile units.
In mobile communications systems, the transmission power level of the base station is controlled in response to the power level of a signal received from the mobile unit so that the signal transmitted from the base station can be received at the mobile unit at a constant power level. With conventional digital modulators, this transmission power control is performed by a variable gain amplifier in the power stage in response to a gain control signal supplied from a power control circuit. This power control circuit includes a selector switch which is responsive to a select command signal to select one of prescribed discrete gain control voltages. If these control voltages are switched from the lowest to the highest value, the switched output would linearly vary in the range of power levels required for compensating for possible base-to-mobile distance variations. Sharp voltage transitions occur at the output of the selector switch as it switches from one level to another. The selected voltage is smoothed by an analog integrator since such transitions would result in the spreading of the frequency spectrum of the transmitted signal if it is directly applied to the variable gain amplifier. However, one disadvantage of the prior art power control circuit is that, due to analog circuit components of the power control circuit manual adjustments must be made before shipment to compensate for variabilities circuit parameters.